The impact of specific perinatal insults is to be intensively studied in a group of infants at unusually high risk of subsequent neurobehavioral abnormality due to extreme prematurity, and/or markedly low birth weight for gestational age. New methods of assessments, sensitive not only to gross cerebral damage, but to emerging cortical function, will be employed to detect injury to brain structures that later subserve higher cognitive processes. These measures include newly developed clinical neurobehavioral assessment and electrophysiologic measures of cortical responses to visual and auditory stimuli. The medical condition of each infant will be evaluated daily and related to neurobehavioral and electrophysiologic studies in an effort to define the timing of adverse effects of post-natal pathophysiology on cerebral functioning. Computerized axial tomography performed at 48 weeks conceptional age will be employed to identify gross cerebral pathology in surviving infants, and neurobehavioral outcome will be evaluated by periodic developmental assessment. The brains of infants who succumb during the neonatal period will be examined by standard neuropathologic techniques, in order to obtain direct morphophysiologic correlations of cortical maturation and structural pathology with the premortem neurobehavioral and electrophysiologic data.